Photon correlation spectroscopy of the phase mode in a ferroelectric liquid crystal under a DC electric field

The phase mode in a ferroelectric liquid crystal has been studied by photon correlation spectroscopy under a dc electric field applied perpendicular to the helical axis of the liquid crystal. The parabolic continuous dispersion of the inverse correlation time τ^-1 against the scattering wave number splits into two branches with a gap at nearly half the wave number at which τ^-1 takes a minimum value. The gap frequency is found to increase in proportion to the applied electric field below the critical field E_c where the helical structure completely unwinds. Above E_c, τ^-1 increases continuously with the wave number and shows no minimum. The experimental findings are compared with the recent predictions based on the phenomenological theory.